Use Ajax technology liberally. Orbitz is a good example. They have the best popup calendar. The site should work equally well on all OS and common browsers - there's no excuse for a website to fully work only on IE these days.

If your airline is a member of an alliance or does a lot of code sharing, let me decide whether to filter out such partners. I may want only a flight on United metal - so don't return screenfulls of US Airways or Lufthansa codeshare flights unless I say it is okay.

As said above, aircraft type, live seat maps, on time performance, and ability to cherrypick fare buckets are very important.

Thanks for the opportunity, and good luck!